1. Field of the Invention
The present invention relates to power processors (converters) generally and more specifically to a non-linear coupling network for use therein, the network for limiting transformer primary winding current during (switching) transitions of a transformer driving potential.
2. Description of the Prior Art
The shunt (winding) capacitance and series (leakage) inductance associated with a transformer present a number of problems, particularly when the transformer is driven by a pulse (or other excitation) having a fast rise and/or fall time. Consider, for example, the related power-processing (converting) circuit configurations which are commonly referred to as half-bridge, full-bridge and push-pull. In the half-bridge circuit configuration, one end of the primary winding of a transformer is connected to the juncture of a pair of voltage-divider capacitors which are connected in series between a pair of terminals for connection to a voltage source, the capacitors for establishing the (capacitor) winding end at the voltage-source-mean potential. The other transformer primary winding end is connected to the juncture of a pair of transistors connected as switches in series between the voltage source terminals. The transistors are driven, in turn, so as to, alternately, couple the (transistor) winding end to each of the voltage source terminals.
The full-bridge circuit configuration employs transistor switches in place of the capacitors of the half-bridge circuit configuration. More specifically, in the full-bridge circuit configuration, one end of the primary winding of a transformer is connected to the juncture of a pair of transistors connected as switches in series between a pair of terminals for connection to a voltage source. The other transformer primary winding is connected to the juncture of another pair of transistors which are also connected as switches in series between the voltage source terminals. The transistors are driven so as to couple the primary winding, in alternate orientations, across the voltage source.
The push-pull circuit configuration employs a center tapped transformer primary winding rather than the capacitors of the half-bridge circuit configuration. Specifically, in the push-pull circuit configuration, a pair of transistors are connected as switches in series between the distal ends of the primary winding of a transformer. The juncture of the transistor switches and the center tap of the primary winding are connected each to a respective one of a pair of terminals for connection to a voltage source. The transistors are driven, in turn, so as to, alternately, couple the voltage source across each of the two transformer primary winding halves.
In each of the above-mentioned power processing (converting) circuit configurations, a transformer is driven by a potential which varies as a series of (alternating polarity) pulses or a square wave. With each change in the transformer driving potential level, the transformer shunt (winding) capacitance is charged/discharged storing/delivering energy. Unfortunately, especially to the extent that the transformer capacitance is charged/discharged during the transistor switching periods, a significant portion of the energy is dissipated in the transistors limiting the performance of the power processor.
Additionally, the combination of the transformer shunt (winding) capacitance and series (leakage) inductance acts as a low-pass filter limiting the response time of the transformer. When the transformer is driven by a pulse having a rise and/or fall time less than that which the transformer can reproduce and when the transformer is not loaded by an impedance equal to or less than the filter characteristic impedance (given by the square root of the leakage inductance over shunt capacitance quotient), a voltage which rings at the (leakage-inductance-shunt-capacitance) resonant frequency is developed across the transformer secondary. In power-processing applications in which the voltage developed across the transformer secondary winding is rectified and filtered by a filter capacitor, the voltage developed across the filter capacitor varies. When lightly loaded, the filter capacitor tends to charge to a potential which approaches the peak (crest) ringing potential. Not only does the power-processor regulation suffer as a result of the peak charging, but the peak potential may prove destructive.
To stabilize the voltage developed across the filter capacitor, it is common practice to employ a bleeder resistor connected across the filter capacitor. Unfortunately, to properly stabilize the voltage developed across the filter capacitor, it is necessary to employ a bleeder resistor having a relatively low resistance. As a consequence, the bleeder resistor dissipates (wastes) a significant amount of power.
A resistor having a suitable resistance may be employed interposed in series with the transformer primary winding to reduce or eliminate ringing and to delay transformer capacitance charging. In addition to limiting the peak transformer capacitance charging current, such a resistor also tends to limit the peak filter capacitance charging current. Unfortunately, to be effective, a relatively large resistance must be employed. As a consequence, a normally intolerable voltage drop is developed across the resistor by the transformer magnitization and/or load current. Further, the power dissipated by the resistor reduces (to often unacceptable levels) the power-processor efficiency.